Speed (especially on the violin, less so on the viola - I chalk it up to the rep being easier) has always been my nemesis. Getting violin showpieces up to tempo has been a bloody, tear-stained battleground. It's still probably one of my top struggles with the instrument. But through (I can finally say this) many years of playing and teaching, I think I've found a few things that help a bit.

1. Practice slowly to play quickly. Practicing in a slow tempo but with attention on quick-twitch actions teaches your muscles to be fast but without the emotional, mental, and physical pressure of having to do clumps of notes. Also, we can often discover sloppy spots (spots we wouldn't necessarily catch if we were plowing through at an ambitious but earless speed) in slow tempi and nip 'em in the bud.

2. This is kind of a fancier versions of #1: Rhythms. This involves playing long-short in various permutations in order to accelerate individual connections in "real time" but with enough under-tempo time to keep your cool. A subset of rhythms, or may be even an equally valued sibling, are playing with stops, or in bursts - ie. a few notes very fast (the "burst") followed by a stop to relax (the relaxation is critical - trains into you the soft, relaxed hands and chill parasympathetic nervous system that you need to play fast for extended periods of time). You can vary the number of notes in your burst, building progressively longer bursts. But if time is a factor, a two note burst can be fantastically effective. The main difference between rhythms and bursts is that, in bursts, your bow actually stops moving.

3. Repeated bows: change bows several times without changing the left hand (in the Suzuki Method, we sometimes call this tool "crickets" or "doubles"). This helps to accelerate the coordination but with enough time between individual changes to prepare the change.

4. Legato: play running notes under one bow - this allows you to focus on the left hand without the distraction of bow changes. Similarly, if it's a legato passage that's troubling you, sometimes practicing with separate bows can help you to see where the left hand may not have a coordinated transition.

5. When all else fails, inch up that metronome incrementally - and with as puny metric subdivisions as you can endure (which will help keep note values even). This is a good way to track progress and recent research shows that keeping the metronome on improves focus.

With continued use of these tools, you should be playing faster...faster! All that said, courage helps! So once you've done the dirty work be BRAVE (that was my personal discovery from alto clef land) and you may surprise yourself:)